1. Description of the Invention
This invention relates generally to an apparatus and method for forming hollow polymer extrudes. More specifically, the invention relates to an apparatus and method for forming hollow cylindrical parisons or extrudes of variable wall thickness. The wall thickness may be varied circumferentially and/or longitudinally over a wide range. The parison shot wall thickness may be simultaneously varied as the parison shot is extruded, providing variable wall thickness in the longitudinal direction.
2. Description of the Invention
During the formation of polymeric structures requiring hollow portions, it is common to extrude heated polymer materials through parison forming devices. These parison forming devices generally consist of an injection system, which receives solid polymer particles, raises the temperature of these polymer particles until they become a viscous liquid, and then forces this viscous liquid under pressure into an extruding die-head.
A general use of such devices is to provide parisons for subsequent forming into finished products, including plastic containers. Blow molding techniques are often employed. The finished products must be structurally capable to accomplish their intended use, but may suffer from extraneous material located in the product walls that undesirably increase the weight and cost. A general aim of the industry is to avoid these shortcomings consistent with the final product configuration. Variations in parison wall thickness often accommodate these configurations.
In one possible situation, it may be preferable for the finished product to have a uniform wall thickness, but the complicated geometry of the blow mold would prohibit such uniformity if a parison of uniform wall thickness were employed. This is in part due to different flow characteristics occurring in different local areas within the mold.
In another situation, it may be preferable for the finished product to have localized areas of increased wall thickness to provide additional strength or rigidity in the finished product, for example around an orifice. A parison of uniform wall thickness would not readily contribute to such variation.
Early attempts to provide parisons of variable wall thickness allowed such variation only longitudinally. The parison wallthickness, at any given longitudinal position along the parison's length or axis, still remained uniform in the circumferential plane.
Others have attempted to provide selective flow restrictions in the injector assembly, upstream of the die-head. These restrictions retard the flow rate of the viscous fluid at selected radial positions before the material enters the die-head. However, remote variation of local flow rates generally allows only a limited range of wall thicknesses, which may not be sufficient for final product structural requirements. Even if the annular opening is increased by forces exerted against the mandrel, which is thereby displaced to provide an annular opening allowing greater or lesser localized flow, this annular opening increase is likewise limited by the elastic limits of the mandrel material. Finally, remote flow restrictions are not able to accommodate wall thickness variations requiring focused local control (i.e., comprising only a few degrees of the circumferential diameter).
Flow restrictions arising from stress applied to external/internal collars or longitudinal movement of external/internal collars is also of limited utility. Displacement of one portion of the collar tends to induce displacement, per the material's Poisson's ratio, of the collar in other locations, thereby changing the flow characteristics in sometimes undesired ways. Further, significant stress is necessary to deflect the internal collars. Finally, the collar must not be stressed beyond its elastic limit.
Separating such collars into segments, which are displaced along the longitudinal axis of the mandrel, may be used to modify local parison shot wall thicknesses. However, it has been found that the effective operating range of longitudinally displaceable segments is limited, due to large displacements of the segments necessary in relation to the increase in gap of the annular opening. Also, the compression zone ratio, defined as the ratio of the compression zone inlet and outlet gap, may vary widely, inducing variations in material flow that may cause cavitation or other undesirable results.
Finally, past devices have not allowed for practical selective localized flow restrictions to occur simultaneously with the actual extrusion process. Many final product configurations require not only circumferential variations, but longitudinal variations at one or more sites. This again facilitates the flow characteristics of complex blow molding configurations, resulting in enhanced strength with lower material usage, weight and cost.